Electronic Structure and Redox Properties of Metal Nitride Endohedral Fullerenes M3N@C2n (M=Sc,Y, La,and Gd; 2n=80, 84, 88, 92, 96)

An extensive study of the redox properties of metal nitride endohedral fullerenes (MNEFs) based on DFT computational calculations has been performed. The electronic structure of the singly oxidized and reduced MNEFs has been thoroughly analyzed and the first anodic and cathodic potentials, as well as the electrochemical gaps, have been predicted for a large number of M₃N@C_2n systems (M=Sc, Y, La, and Gd; 2n=80, 84, 88, 92, and 96). In particular, calculations that include thermal and entropic effects correctly predict the different anodic behavior of the two isomers (I_h and D_5h) of Sc₃N@C₈₀, which is the basis for their electrochemical separation. Important differences were found in the electronic structure of reduced M₃N@C₈₀ when M=Sc or when M is a more electropositive metal, such as Y or Gd. Moreover, the changes in the electrochemical gaps within the Gd₃N@C_2n series (2n=80, 84, and 88) have been rationalized and the use of Y‐based computational models to study the Gd‐based systems has been justified. The redox properties of the largest MNEFs characterized so far, La₃N@C_2n (2n=92 and 96), were also correctly predicted. Finally, the quality of these predictions and their usefulness in distinguishing the carbon cages for MNEFs with unknown structures is discussed.     

Thermodynamic analysis of LiF-BeF2 and KF-BeF2 melts by a structural model

An earlier structural model for binary silicate melts and glasses is extended to MF-BeF₂ (M = Li, K) systems. The evaluation of the thermodynamic properties as well as the phase diagrams for the binary LiF-BeF₂ system and the integral enthalpy of mixing of the KF-BeF₂ system are carried out with this model. This thermodynamic model is based on the assumption that each alkali fluoride produces the depolymerization of BeF₂ network with a characteristic free energy change. A least squares optimization program permits all available thermodynamic and phase diagram data to be optimized simultaneously. In this manner, data for these binary systems have been analysed and represented with a small number of parameters. The model predicts the chain-length distribution of polymeric ions, even though these are not explicitly treated as structural units of the model. The calculated fluoride polyanion chain-length distribution for the LiF-BeF₂ system is in quantitative agreement with the predictions reported in the literature.     

Structural studies of cyclic ureas: 3. Enthalpy of formation of barbital

A thermochemical and thermophysical study has been carried out for crystalline barbital [5,5′-diethylbarbituric acid]. The thermochemical study was made by static bomb combustion calorimetry, from which the standard () molar enthalpy of formation of the crystalline barbital, at T = 298.15 K, was derived as −(753.0 ± 1.8) kJ · mol⁻¹. The thermophysical study was made by differential scanning calorimetry over the temperature interval (265 to 470) K. A solid–solid phase transition was found at T = 413.3 K. The vapour pressures of the crystalline barbital were measured at several temperatures between T = (355 and 377) K, by the Knudsen mass-loss effusion technique, from which the standard molar enthalpy of sublimation, at T = 298.15 K was derived as (117.3 ± 0.6) kJ · mol⁻¹. The combination of the experimental results yielded the standard molar enthalpy of formation of barbital in the gaseous phase, at T = 298.15 K, as −(635.8 ± 1.9) kJ · mol⁻¹. This value is compared and discussed with our theoretical calculations by several methods (Gaussian-n theories G2 and G3, complete basis set CBS-QB3, density functional B3P86 and B3LYP) by means of atomization and isodesmic reaction schemes.     

Nir-chemical imaging study of acetylsalicylic acid in commercial tablets

Near Infrared Chemical Imaging (NIR-CI) is demonstrating an increasing interest in pharmaceutical research since it meets the challenging analytical needs of pharmaceutical quality and may serve as a versatile adjunct to conventional NIR spectroscopy in many fields.The direct analysis of samples by using hyperspectral imaging techniques, which provide a NIR spectrum in each pixel of the image, generates a big amount of information from one sample. Focusing the interest in pharmaceutical research, several chemometric algorithms are demonstrating their usefulness extracting the relevant information (i.e. quantitative determination of the component in one sample) in tablets with only one sample and without damaging it.In this work, a quantitative method to analyze different commercial Acetylsalicylic acid tablets is proposed by using Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) method to the hyperspectral image and without any previous calibration model. For this purpose, a large concentration range of active pharmaceutical ingredient (ASA, Acetylsalicylic acid in this work), between 82% and 12%, was covered depending on the manufacturer. MCR-ALS allowed obtaining a concentration maps for acetylsalicylic acid and therefore, consequent analysis of the ASA distribution in the tablet was developed by using the histograms of the distribution of concentration.Results certified the good distribution of ASA despite the different origins of the tablets. Moreover, the obtained values of concentration showed a very good concordance with the nominal value of ASA. As a matter of fact, the quality of the results demonstrated the useful of encompassing NIR-CI techniques with MCR-ALS and, consequently, the well development on the production of Acetylsalicylic acid tablets.     

Hollow fiber-based liquid-phase microextraction (HF-LPME) of ibuprofen followed by FIA-chemiluminescence determination using the acidic permanganate-sulfite system

Hollow fiber-based liquid-phase microextraction (HF-LPME) is a relatively new technique employed in analytical chemistry for sample pretreatment which offers more selectivity and sensitivity than any traditional extraction technique. This paper describes a three-phase HF-LPME method for ibuprofen using a polypropylene membrane supporting dihexyl ether followed by a chemiluminescence (CL) determination using the CL enhancement on the acidic permanganate-sulfite system in a FIA configuration which is the first time that both techniques have been combined for analytical purposes. The CL intensity (peak area) was proportional to the log of ibuprofen concentration in the donor phase over the range 0.1-20 μg mL⁻¹. The detection limit was 0.03 μg mL⁻¹ of ibuprofen in the donor phase. The method was satisfactory reproducible and has been applied to the ibuprofen determination in pharmaceuticals and in real human urine samples.     

Leptogorgolide, a biogenetically interesting 1,4-diketo-cembranoid that reinforces the oxidation profile of C-18 as taxonomical marker for octocorals

The cembranoid 1 and the furanocembranolides 2-4 along with the known pukalide were isolated from Leptogorgia sp. and their structures determined spectroscopically. The 1,4-diketo-cembranoid 1 follows an oxidation pattern of C-18 that reinforces the concept of oxidation profile of C-18 as taxonomical marker for octocorals. The co-occurrence within a species of furanocembranolide/1,4-diketo-cembranoid congeners 1/2-4 raises the question about which one is the biogenetic precursor. A biogenetic pathway is proposed.     

A theoretical study of diborenes HLB=BLH for L=CO, NH3, OH2, PH3, SH2, ClH: structures, energies, and spin–spin coupling constants

Ab initio calculations were carried out to investigate the structures, binding energies, bonding, and NMR spin–spin coupling constants of complexes HLB=BLH, for L=CO, NH₃, OH₂, PH₃, SH₂, and ClH. Both B–B and B–H bonds lengthen on complex formation relative to singlet HBBH, and except for L=CO, the B–B bonds are double bonds. The order of stability of the trans isomers correlates with the ordering of ligands in the spectrochemical series of ligand field theory. The trans isomer is always more stable than the corresponding cis. Inverse correlations are found between ¹ J(B–B) and ¹ J(B–H) and the corresponding B–B and B–H distances. For the trans isomers, ¹ J(B–B) appears to be related to the ordering of ligands in the spectrochemical series, while ¹ J(B–H) is related to the protonation energy of the ligand L.     

Mechanism of the Catalytic Activity of Nucleophiles in the Stepwise Hydrolysis and Condensation Reactions of Tetramethoxysilane

Active ingredients : A model for the simplest hydrolysis reaction is applied to all stages of stepwise hydrolysis and condensation taking place during a sol–gel process. The picture shows the molecular structures of the transition states of the ammonia‐ (left) and OH^?‐promoted (right) condensation reactions of two Si(OH)₄ molecules, including an additional water molecule.